Determining the temperature of an exhaust gas sensor by means of calibrated internal resistance measurement

ABSTRACT

For achieving a highest possible measuring accuracy of a circuit for measuring the internal resistance of an exhaust-gas sensor, a method and a circuit for measuring the internal resistance R 1  ( 10 ) of an electrochemical cell ( 12 ) are provided for determining the temperature of an exhaust-gas sensor, especially of a motor vehicle. The circuit is provided with the objective to improve the control to a constant temperature to therefore also improve the performance of the exhaust-gas sensor. A measurement current I_Mess ( 20 ) is applied to the internal resistance R 1  ( 10 ) of the electrochemical cell ( 12 ) and a resulting first voltage is detected. A switchover to a reference resistor R 2  takes place from time to time or at regular time intervals. With a switchover to the reference resistor R 2,  the resulting second voltage is stored and thereafter is applied as a reference value for the measurement of the internal resistance R 1  ( 10 ). The switchover ( 18 ) to the reference resistor R 2  takes place, for example, via force/sense lines.

DESCRIPTION

[0001] The invention relates generally to the temperature measurement inexhaust-gas sensors, especially of motor vehicles and, especially, amethod and a circuit for measuring the internal resistance of anelectrochemical cell determining the temperature of such an exhaust-gassensor.

[0002] A lambda control in combination with a catalytic converter istoday the most effective exhaust-gas cleaning method for thespark-ignition engine. Very low exhaust-gas values can be obtained onlytogether with ignition and injection systems which are presentlyavailable. The use of a three-way or selective catalytic converter isespecially effective. This catalytic converter type has thecharacteristic of decomposing hydrocarbons, carbon monoxide and nitrousoxide up to more than 98% in the event that the engine is operated in arange of approximately 1% about the stoichiometric air/fuel ratio withlambda=1. The lambda indicates how far the actually present air/fuelmixture deviates from the value lambda=1, which corresponds to the massratio of 14.7 kg air to 1 kg gasoline, which is theoretically necessaryfor the complete combustion; that is, lambda is the quotient of suppliedair mass and theoretical air requirement.

[0003] In the lambda control, the particular exhaust gas is measured andthe supplied fuel quantity is immediately corrected in correspondence tothe measurement signal by means of, for example, the injection system. Alambda probe is used as a measurement sensor which exhibits a voltagejump exactly at lambda=1 and so supplies a signal which indicateswhether the mixture is richer or leaner than lambda=1. The operation ofthe lambda probe is based on the principle of a galvanic oxygenconcentration cell having a solid-state electrolyte.

[0004] Lambda probes, which are configured as two-point sensors, operatein accordance with the Nernst principle as known per se based on aNernst cell. The solid-state electrolyte comprises two boundary surfacesseparated by a ceramic. The utilized ceramic material becomes conductivefor oxygen ions at approximately 350° C. so that the so-called Nernstvoltage is generated for a different oxygen component at both sides ofthe ceramic between the boundary surfaces. This electrical voltage is anindex for the difference of the oxygen component at both sides of theceramic. The residual oxygen content in the exhaust gas of an internalcombustion engine is dependent to a great extent on the air/fuel ratioof the mixture supplied into the engine. For this reason, it is possibleto apply the oxygen content in the exhaust gas as an index for theair/fuel mixture actually present.

[0005] The function and the measuring accuracy of the lambda probes isdependent, to a very great extent, on the temperature of the measuringelement, that is, on the Nernst cell in the present case. The probetemperature would be subjected to intense fluctuations withoutadditional measures because of the changing exhaust-gas temperatures andexhaust-gas quantities. Accordingly, in a manner known per se, the probetemperature is held as constant as possible. This controlled power issupplied to the probe with the aid of an electrical heater. A suitablemeasurement signal, which indicates the sensor temperature, is needed inorder to determine the particular required quantity of heating power. Asa rule, the electric internal resistance of the electrochemical Nernstcell is applied as a measurement signal. For this purpose, for example,a measurement current is applied to the internal resistance and thevoltage which adjusts is determined with the aid of an evaluationcircuit.

[0006] The measurement current is preadjusted via suitable dimensioningof the evaluation circuit in a manner known per se. In the components ofthe evaluation circuit, often tolerances which are present lead to faultinfluences in the measurement of the above-mentioned internal resistanceand thereby affect the control accuracy of the heater control.

[0007] It is therefore a basis of the present invention to provide amethod initially mentioned herein as well as a circuit which avoid theabove-mentioned disadvantages and make available the highest possiblemeasuring accuracy of a circuit for measuring the internal resistancewith the objective of improving the control to a constant temperatureand therefore improving also the performance of the exhaust-gas sensor.

[0008] This task is solved with the features of the independent claims.Advantageous embodiments or further improvements are the subject matterof the dependent claims.

[0009] The invention suggests a special calibration method wherein(preferably in combination with force/sense lines) there is a switchoverfrom time to time or regularly to a reference resistance and theelectric voltage, which then adjusts, is stored in a memory. This storedvoltage value thereafter serves as a reference value for the measurementof the actually desired value of the internal resistance R1.

[0010] With these measures, the measuring accuracy of a circuit formeasuring the internal resistance of an exhaust-gas sensor can beincreased. The method of the invention as well as the circuit facilitateespecially the system performance of the composite of exhaust-gas sensor(for example, lambda probe) and the evaluation circuit mentionedinitially herein.

[0011] The invention will be explained hereinafter with reference to theattached drawing and with reference to an embodiment. The single FIGUREshows a block circuit diagram of a circuit according to the invention.

[0012] The shown circuit functions to measure the internal resistance(R1) 10 and therefore serves indirectly for determining the temperatureof a schematically shown Nernst cell 12 of an electrochemicalexhaust-gas sensor (not shown). The electrochemical source voltage ofthe Nernst cell is here identified by 12 (U1). The circuit comprisesthree circuit units: a measurement current generating unit 14; ameasurement signal evaluation unit 16; and, a switchover unit 18.

[0013] The generated measurement current (I_Mess) 20 is applied to theNernst cell 12. By means of this measurement current (I_Mess) 20, avoltage is generated at R1 which is proportional to the resistance valueof R1. This voltage is then amplified by the measurement signalevaluation unit 16 and is so processed that an optimal detection of themeasurement signal is made possible via an analog-to-digital converter(not shown). This signal can then be advantageously further processeddigitally with the aid of the signal supplied by the analog-to-digitalconverter.

[0014] Possibly occurring tolerances and voltage drifts in themeasurement current generating unit 14 as well as in the measurementsignal evaluation unit 16 would, without special measures, be includedas errors in the output signal (UA). To obtain a high accuracy of the R1signal, highly precise and therefore expensive circuit techniques wouldhave to be used.

[0015] The circuit shown as well as the method for operating the samemake possible an improvement of accuracy exclusively with the use ofelectronic standard components. In addition, an integration of thecircuit by means of standard semiconductor processes is made possible.

[0016] On the one hand, the calibration resistance R2 and the switchoverunit 18, which includes several throwover switches (S1 to S4), serve forthe above. By means of the throwover switches S1 to S4, the measurementvalue detection is switched over from time to time or at regular timeintervals to the known, precisely defined resistor R2. The resistancevalue of R2 is so selected that it corresponds to the internalresistance R1 to be adjusted (in correspondence to the control point ofthe heater control). The signal voltage UA then adjusts at the output ofthe circuit and is stored in a memory of a microcontroller (not shown)and serves from thereon as a reference value for the measurement of theinternal resistance R1.

[0017] With this measure, circuit-caused defects in the measurementcurrent generating unit 14 and the measurement signal evaluation unit 16can be eliminated. The accuracy of the output signal UA is thereforedetermined only by the accuracy of the calibrating resistor R2.

[0018] In order to further eliminate additional errors from thethrowover switches S1 to S4, the switchover unit 18 is designed in thepresent embodiment as a force/sense circuit. The measurement currentI_Mess 20 is switched via switches S1 and S2 (force switches) to themeasuring resistor R1 or R2. Only slight accuracy requirements areimposed on the switches S1 and S2. Only the simultaneous operation ofall switches has to satisfy minimal requirements which, as a rule, iseasily satisfied for an integration of the circuit, for example, into anapplication specific integrated circuit (ASIC).

[0019] It is to be noted that the force/sense circuit is not absolutelyrequired and can be omitted, for example, when the ratio of theresistance to be measured to the switch resistances is sufficiently highas, for example, with the use of low-ohmage switches. The same applieswhen the switches supply significant contributions to the measurementresistance value but the absolute value of the resistance to be measuredis not important but it is only important to come as close as possibleto the comparator resistor R2 which is present.

[0020] Coupling in the measurement signal into the measurement signalevaluation unit 16 takes place via the two switches S3 and S4 (senseswitches). The measurement signal is only minimally influenced becauseof the high-ohmage input of the measurement signal evaluation unit 16.For this reason, relatively simple and cost effective (high ohmage)switches can be used for S3 and S4.

[0021] Measuring errors caused by the switches S1 to S4 are eliminatedvia the described calibration by means of resistor R2 as long as only S1and S2 or S3 and S4 have comparable characteristics, for example, thesame or like through-switch resistance.

[0022] Finally, it is noted that the evaluation circuit of the inventioncan also be advantageously used for two-cell broadband lambda probeswhich are formed from a Nernst cell and a pump cell coupled to thelatter. The lambda probe and the evaluation circuit together provide acontinuous lambda signal by means of which a lambda control can beadjusted to any desired operating point, that is, also to lambda unequalto 1 and, in this way, a “continuous lambda control” is provided.

1. Method of measuring the internal resistance R1 (10) of anelectrochemical cell (12) for determining the temperature of anexhaust-gas sensor having an electrochemical cell, especially of a motorvehicle, characterized in that: a measurement current I_Mess (20) isapplied to the internal resistance R1 (10) of the electrochemical cell(12) and a resulting first voltage is detected; from time to time or atregular time intervals, there is a switchover to a reference resistorR2; and, a second voltage is stored and thereafter is applied asreference value for the measurement of the internal resistance R1 (10),the second voltage resulting from the measurement current when there isa switchover to the reference resistor R2.
 2. Method of claim 1,characterized in that the switchover to the reference resistor R2 takesplace by means of force/sense lines.
 3. Method of claim 1 or 2,characterized in that a voltage signal, which results at internalresistance R1, is amplified and/or processed by means of a measurementsignal evaluation unit (16).
 4. Method of claim 3, characterized in thatthe amplified and/or processed voltage signal is supplied to ananalog-to-digital converter.
 5. Method of one of the claims 2 to 4,characterized in that the measurement current I_Mess (20) is switched tothe measuring resistors R1 and R2 by means of force switches S1, S2 andthat the in-coupling of the detected measurement voltage into themeasurement signal evaluation unit (16) takes place via sense switchesS3, S4.
 6. Method of claim 5, characterized in that the switches S1 andS2 as well as the switches S3 and S4 have the same or a similarthrough-switch resistance.
 7. Method of one of the above claims,characterized in that the resistance value of R2 is so selected that itessentially corresponds to the internal resistance R1 which is to beadjusted.
 8. Circuit for measuring the internal resistance R1 (10) of anelectrochemical cell (12) for determining the temperature of anexhaust-gas sensor, especially of a motor vehicle, characterized bycircuit means for carrying out the method of one of the above claims. 9.Circuit of claim 8, characterized by a reference resistor R2, aswitchover unit (18) having several throwover switches (S1 to S4); and,the measurement value detection can be switched from time to time orregularly to the reference resistor R2 by means of the throwoverswitches (S1 to S4).
 10. Circuit of claim 8 or 9, characterized by ameasurement current generating unit (14) and a measurement signalevaluation unit (16); the switchover of the measurement current I_Mess(20) to the measurement resistors R1 and R2 and the in-coupling of adetected measurement voltage into the measurement signal evaluation unit(16) takes place via force/sense lines.